The invention applies in particular to the manufacture of bars from the refractory composite materials proposed by Applicant in French Pat. No. 69,12452, in Patent of Addition No. 69,44707, in French Pat. No. 74,31140 and in French patent application No. 78,32151 (see U.S. Pat. Nos. 4,175,609, 3,973,750, 3,871,835, 3,985,582, 4,043,841) and which comprise a complex matrix made of a nickel- and/or iron-, and/or cobalt-based superalloy containing chromium as well as eventually other elements such as tungsten and aluminum and in which is present a reinforcement phase made of monocrystalline fibers of at least one transition metal monocarbide. Due to their very good mechanical properties, such materials are particularly suitable as constituent materials for parts subject in operation to high stresses at high temperatures, such as the blades of aircraft turbines.
Such parts are manufactured either from rough casts or ingots in which is machined the required part, for instance a turbine blade, or directly by casting, the alloy being solidified in a mold having substantially the shape of the required part. In the processes used hitherto, the alloy which has to be subjected to the unidirectional solidification is previously introduced at one time only into the mold, either by casting or else in the form of a prealloyed powder, viz. a powder the grains of which are substantially identical and have the nominal composition of the alloy. The mold is then shifted relative to a hot source and a cold source superimposed, the distance between the hot source and the cold source as well as the efficiency of the hot source on the one hand, and the displacement speed of the mold on the other hand, being set so that within the alloy contained in the mold is formed a rigorously planar solidification front with a high thermal gradient at the level of said front, and so as to thereby obtain grains and reinforcement flakes or fibers perpendicular to the solidification front. For materials with monocrystalline fibres of monocarbides like those of the aforementioned patent, the thermal gradient which is established is of the order of 120.degree. to 200.degree. C./cm at the solidification front and the mold is moved at speeds of the order of 1 cm/h.
Hitherto, it has never been possible to manufacture parts of a great length exhibiting over their whole length constant mechanical properties. In fact, and due to the great height of the liquid portion contained in the mold, there appears a segregation phenomenon connected to the convection movements of the liquid, which phenomenon is well known to metal founders. At the level of the solid liquid interface, the constituent elements of the alloy are unequally distributed between the solid phase and the liquid phase, in accordance to their respective coefficients of parting. Thus, for instance, the chromium is incorporated to the solid in formation in a smaller proportion than its proportion in the liquid phase, the latter having therefore the tendency to become richer in chromium in the vicinity of the solidification front; on the contrary, the tungsten is preferentially incorporated to the solid in formation in a higher proportion than its proportion in the liquid phase, the latter growing poorer in tungsten. The result is that, when one wishes to manufacture a large size part and when the mold contains in consequence, at the beginning of the solidification process, a liquid portion of great height, the solidified material which is obtained as the solidification progresses unavoidably exhibits composition variations between the portion which was solidified in the first place and the portion which was solidified in the last place. As regards the particular elements which are considered here by way of example, the latter portion exhibits a chromium content and a tungsten content which are, respectively, substantially higher and lower than the contents present in the portion which was first solidified. The result is that the metallurgical structure is not constant from one end to the other of the manufactured material, the volume fraction of the reinforcement fibers within the matrix, considered in the successive cross sections of the material, altering from one end to the other due to the concentration evolution of the constituent elements of the fibers with, as a consequence, a variation of the mechanical properties of the solidified part.